There are known in the art power steering apparatus for generating a steering assisting force to allow the driver of a vehicle to steer the vehicle with a reduced steering force.
Japanese Laid-Open Patent Publication No. 54-110527 discloses a hydraulic power steering apparatus including a preloading device for use on a vehicle. The preloading device generates a hydraulic pressure against a spring force which presses a rack guide at all times and reduces a pressing force applied to a pinion, i.e., a steering shaft, to lower the hydraulic pressure when a steering wheel is turned, so that the rotational resistive force applied to the steering wheel when the steering wheel is turned is smaller than the rotational resistive force applied to the steering wheel when the steering wheel is at rest.
Japanese Laid-Open Patent Publication No. 60-174364 discloses a hydraulic power steering apparatus including a hydraulic pressing mechanism for use on a vehicle. The hydraulic pressing mechanism applies a hydraulic pressure to a rack guide depending on the speed of the vehicle only in a neutral region of the steering wheel, thereby preventing the steering wheel from wobbling at the time the vehicle travels at high speeds. The hydraulic pressing mechanism also applies a frictional resistive force depending on the speed of the vehicle to a rack shaft to reduce vibrations that are transmitted from road wheels to the steering wheel. When the steering wheel is turned out of the neutral region, the hydraulic pressing mechanism stops applying the frictional resistive force to the rack guide to allow the driver to turn the steering wheel with reduced manual forces.
Japanese Laid-Open Patent Publication No. 2000-313348 discloses a hydraulic power steering apparatus for use on a vehicle. The disclosed hydraulic power steering apparatus includes a rack guide for pressing a rack toward a pinion under a hydraulic pressure, a spring for applying a constant pressing force to the rack guide at all times, and a preloading device for increasing the pressing force applied by the rack guide depending on an increase in the hydraulic pressure. When the steering wheel of the vehicle is turned, the hydraulic pressure is lowered to reduce the pressing force applied by the rack guide to allow the driver of the vehicle to turn the steering wheel with reduced manual forces. When the steering wheel is in its neutral position, the hydraulic pressure is increased to increase the pressing force applied by the rack guide for higher steering stability.
Generally, if a frictional resistive force (hereinafter referred to as “friction”) acting on the steering system, including a steering wheel, of a vehicle is large, then disturbances from the road are suppressed, allowing the driver of the vehicle to turn the steering wheel stably and hence steer the vehicle with ease.
Therefore, to attain better steering stability and comfortableness while the vehicle is being driven, it is preferable for the friction applied to the steering wheel to be relatively large at the time the steering wheel is held still when the vehicle is traveling along a curve having a constant turning radius (in other words, when the steering wheel is held to keep the vehicle turning) or when the vehicle is traveling straight.
If it is assumed that the gain of a steering assisting force (referred to as “assisting gain”) of a power steering system is indicated by GA, then at the time the steering assisting force is acting when the steering wheel is held to keep the vehicle turning, the steering stabilizing capability based on friction is canceled by the steering assisting force and reduced to 1/(1+GA).
In order for the power steering system to achieve a desired steering stabilizing capability in such a situation, therefore, more friction needs to be applied to the power steering system.
On the other hand, when the steering wheel is turned to steer the vehicle, it is preferable to reduce a friction loss, and when the steering wheel is turned back, the fiction acting on the steering wheel should preferably be relatively small.
According to the technologies disclosed in Japanese Laid-Open Patent Publication No. 54-110527, Japanese Laid-Open Patent Publication No. 60-174364, and Japanese Laid-Open Patent Publication No. 2000-313348, the magnitude of the friction acting on the steering wheel can be adjusted by varying the pressing force applied by the rack guide.
According to the technologies disclosed in Japanese Laid-Open Patent Publication No. 54-110527 and Japanese Laid-Open Patent Publication No. 60-174364, however, since the friction cannot be increased when the steering wheel is held to keep the vehicle turning, no steering stability can be attained when the steering wheel is held to keep the vehicle turning.
According to the technologies disclosed in Japanese Laid-Open Patent Publication No. 54-110527, Japanese Laid-Open Patent Publication No. 60-174364, and Japanese Laid-Open Patent Publication No. 2000-313348, as the friction is mechanically increased, the steering assisting mechanism has to generate an increased amount of work commensurate with the mechanically increased friction. Consequently, the problem of an increased friction loss cannot be solved.
One solution is to apply friction electrically rather than mechanically.
Japanese Patent No. 3840310 discloses an electric power steering apparatus that is designed based on the idea that in order to cancel friction acting in a direction which runs counter to the rotational direction of a motor, the motor may generate output power in the same direction as the rotational direction of the motor. According to the disclosed electric power steering apparatus, the rotational direction of the motor is detected by a motor angular velocity sensor, and the polarity of a friction compensating value is selected in order to enable the motor to generate output power in the same direction as the rotational direction of the motor.